Normal phase SPE uses polar interactions as retention mechanism. These interactions include hydrogen bonding, dipole-dipole, induced dipole, π-π, etc. The interactions are between polar functional groups of analytes and polar sorbent chemistries.
Reversed phase uses nonpolar, hydrophobic interactions as retention mechanism. These interactions include Van-der-Waals, London dispersion, π-π, etc. These interactions are between carbon rich, hydrophobic parts of both analytes and sorbent.
When dealing with sample matrices that are only soluble in organic solvents, one would use a normal phase method. Examples of such matrices are oil, grease, or butter.
Ideally, the sample is diluted with a nonpolar solvent (i.e. hexane) prior to sample loading. The sample can then be washed with hexane or 5% methylene chloride in hexane (analogous to washing with 5% MeOH in water in RP SPE methods to remove matrix interferences). Elution will be done with a polar solvent that can interrupt polar interactions between analytes and sorbent.
Fractionation can be done by applying solvents in the order of increasing polarity or mixtures of solvents with different polarities.
Here is a quick reference of solvents in ascending polarity for NP SPE methods:
Hexane
Isooctane
Carbon tetrachloride
Toluene
Benzene
t-butyl methyl ether
Chloroform
Dichloromethane
Diethyl ether
Ethyl acetate
Tetrahydrofuran (THF)
Acetone
Acetonitrile
40% MeOH in acetonitrile
20% MeOH in diethyl ether
20% MeOH in dichloromethane
Isopropanol
Methanol
Water
Acetic acid
0 Comments